Investigators seeking to improve the accuracy of force-measuring devices such as weighing scales are called upon to consider and attempt compensation for a variety of phenomena such as drift, anelastic creep, temperature induced effects, and hysteresis. The latter, hysteresis phenomenon, has posed significant problems for many years and finds its origin in the load cells or transducers of force measuring devices. Typically, a force measuring device will be configured having a weight receiving platform supported, in turn, by a load cell incorporating a counterforce or structure stressed by and responding in strain between loads applied to the platform and the device or scale ground. Force measuring instrumentation of the load cell, such as strain gauges and the like, react to such strain to provide an output, typically requiring correction for the above phenomena. Hysteresis, or as it is sometimes called, "Internal Friction" appears in the output of the transducer when a series of loads are applied and then removed in any of a myriad of sequences. In a typical course of scale usage these sequences of loading, for the most part, are of a random nature, for example, loads extending in value from partial to full being added and either fully or only partially removed from the scales. Comparing the outputs or readings of the devices as such loads are removed with the readings, at the same load, as the loads are being applied, results in a difference. This difference is considered to be hysteresis. The hysteresis effect arises primarily from the material properties and geometry of the weighing apparatus transducer. For example, all counterforce materials, whether metal or glass/ceramic exhibit hysteresis to varying degrees. Conventional, epoxy based strain gauges, as are used extensively in weighing devices, also exhibit hysteresis and contribute significantly to the overall hysteresis seen in scale outputs. Apart from strain gauge generation of the effect, the hysteresis phenomenon may also arise from the method of mounting or applying a load to the scale transducer. In this regard, there often is a slippage or movement between the load cell and the scale structure which is manifested as hysteresis.
A conventional approach to improving the hysteresis effect has been to improve the quality of the counterforce or transducer material. For example, lower levels of hysteresis are exhibited by forming the load cells of such materials as beryllium-copper or glass/ceramic materials. However, these approaches are considered overly-expensive for employment with scale structures intended for conventional utilization. The latter materials also are subject to certain manufacturing difficulties. Improvement in hysteresis has been achieved for certain applications by varying production parameters and heat treating procedures. However, these techniques are heuristic in nature and essentially non-repeatable from component to component. While a hysteresis effect will appear in the strain gauges of load cell instrumentation, other sensors such as those representing the vibrating wire or capacitor technique do not exhibit hysteresis in and of themselves but the phenomenon will appear in the output of cells having such instrumentation, inasmuch as it remains within the overall counterforce structure.
With the advent of microprocessor driven instrumentation, a practical approach to this phenomenon will be to achieve a predictive method for digital correction of hysteresis at the load cell output. However, this approach requires an accurate quantification of the hysteresis based behavior of load cells. Such a predictive digital correction approach has been successfully introduced with respect to creep pheomenon. See in this regard U.S. Pat. No. 4,691,290 entitled "Creep-Compensated Weighting Apparatus" by Griffen, issued Sep. 1, 1987, and assigned in common herewith.